16638025

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Infrared spectra of two sexithiophenes in neutral and doped states:

a theoretical and spectroscopic study

J. Casado

a,c

, H.E. Katz

b

, V. Herna´ndez

a

, J.T. Lo´pez Navarrete

a,* a

Departamento de Quı´mica Fı´sica, Facultad de Ciencias, Universidad de Ma´laga, 29071 Ma´laga, Spain b

AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974, USA c

Department of Chemistry, University of Minnesota, Minneapolis, MN 55455, USA

Received 19 October 2001; received in revised form 21 February 2002; accepted 4 March 2002

Abstract

The FT-infrared spectra of two sexithiophenes having their enda,a0_{-positions substituted by}_n_{-hexyl or -thiohexyl groups, in}

neutral and doped states, are studied with the main aim of deriving information about thep-electrons delocalization and about the electronic structure of the charged defects created upon doping with iodine. The analysis of the experimental data is aided by Density Functional Theory calculations. The modifications in the electronic structure of the sexithiophene backbone induced by the n-thiohexyl encapsulation are discussed from the point of view of single molecule interactions in thiol-terminated

p-conjugated oligomers bound to metallic or cluster electrodes.

Keywords:Oligothiophenes; Infrared spectroscopy;p-Electron interactions; Chemical doping; Radical cation; Theoretical calculations

1. Introduction

Vibrational spectroscopy is among the most impor-tant and promising techniques for the characterization of organic polyconjugated polymers and oligomers, both in the undoped and doped states. Vibrational spectra of p-conjugated materials constitute a very rich source of information about their molecular structure, charge distribution and conjugational prop-erties[1,2]. In particular, infrared and Raman spectra of polyconjugated chain compounds show peculiar and characteristic features directly related to the effi-ciency of the p-electrons delocalization along the quasi one-dimensional path of alternating C=C/C–C

bonds and also with the different types of charged defects created upon chemical doping or photoexcita-tion [3–5]. In this regard, we must stress that the attainment of detailed information on the microstruc-ture of the doped materials in terms of bond lengths and bond angles is hardly accesible by means of other experimental techniques. An alternative way to obtain this type of structural information is to combine vibrational spectroscopies with theoretical calcula-tions [6–10].

Polythiophene is among the most thoroughly inves-tigated polyconjugated polymers [11,12]. However, polythiophene samples synthesized so far have the traditional complexity of ‘‘real’’ polymers such as their low solubility, high contents of structural defects, broad distribution of molecular weights, etc. The difficulties inherent to the synthesis of any structurally well-defined p-conjugated polymer led to numerous

Vibrational Spectroscopy 30 (2002) 175–189

*_{Corresponding author. Tel.:}_þ_{34-952-132-081;} fax:þ34-952-132-000.

E-mail address:[email protected] (J.T. Lo´pez Navarrete).

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attempts of obtaining their low molecular weight counterparts[13–16]. Over the last decade, it has been possible to assess precise relationships between the physico-chemical properties of these p-conjugated materials and their chemical architectures, starting from the systematic study of different series of oligo-mers with variable chain lengths. This strategy is commonly known as the ‘‘oligomeric approach’’ [17]. On the other hand, oligothiophenes have been already used as active components in electronic devices such as field effect transistors (FETs) and light emitting diodes (LEDs)[18,19].

Likely, future computers will consist of logic devices that are ultradense, ultrafast and molecular-sized [20,21]. The transmision times could be minimized by using molecular scale electronic inter-connects, thus resulting in computational systems that operate at far greater speeds[22]. Alligator clips are moieties that allow for the connection of single mole-cules (i.e. oligothiophenes) to a macroscopic interface, usually a metallic tip or a nanoscale cluster. The characterization of the tip/molecule interface is of crucial importance in the design of these molecular electronic circuits. The majority of systems studied so far use thiol-terminated molecules, because of sulfur’s ability to bond to a great variety of metal surfaces [23,24]. In this context, the a,a0-(n-thiohexyl) end-capped sexithiophene studied in this paper, referred to as DHTSxT henceforth, can be viewed as a surface-bound sexithiophene bearing two end thioether (SR) substituents, where the alkyl groups play the role of the tip while the S atoms act as the alligator clips.

Current quantum-chemical methods are in the posi-tion to give reliable informaposi-tion about the molecular structure and vibrational properties of the different classes of polyconjugated materials. Most current calculations are performed within the ab initio Har-tree–Fock (HF) scheme. At this level of theory, the calculated harmonic vibrational frequencies are usually higher than the corresponding experimental quantities, due to electron correlation effects and basis set deficiencies. Density functional theory (DFT) con-stitutes a non-expensive approach for adding electron correlation, being its computational requirements comparable to those of the HF method. DFT studies have been probed very useful in the study of charged molecules or ions[25–27]. Recently, the spin-unrest-ricted DFT methods have been successfully applied to

the study of the polaron to bipolaron transition in oligophenyls [27]. Our theoretical work is based on the use of the DFT methodology to calculate ground-state geometries as well as vibrational frequencies and intensities for model oligothiophenes.

We have previously reported a spectroelectrochem-ical Raman and theoretspectroelectrochem-ical study of these two end-capped sexithiophenes, both in their neutral and doped forms[28]. The doping process was found to generate two stable oxidized species: a radical cation type defect at low anodic potentials and a dication type defect at high potential values. In order to achieve a more detailed information on the electronic charge distribution and the effects of then-hexyl and -thio-hexyl-substitution, we report here a new theoretical and infrared spectroscopic study of the above hexam-ers. The analysis of the experimental spectra will be guided by means of quantum-chemical calculations carried out on two quaterthiophenes,a,a0-end-capped by n–propyl groups, DPQtT, and by n-thiopropyl groups, DPTQtT, as model systems for DHSxT and DHTSxT, respectively.

2. Experimental and computational details

The two sexithiophenes were prepared following a procedure described elsewhere [29]. The chemical structures of DHTSxT and DHSxT are displayed in Fig. 1, together with that of the a,a0-dimethyl end-capped sexithiophene (DMSxT) for comparison pur-poses. Although the DMSxT compound has been already studied in depth, it will be referred to as a model compound bearing a shorta-alkyl side chain [30]. The chemical doping of the compounds was carried out, under dry atmosphere, by slow in situ sublimation of iodine at room temperature using a solid–vapor doping technique.

FT-infrared measurements were made with a Per-kin-Elmer Model 1760X spectrometer, on the pure and iodine-doped solid compounds, in the form of KBr pellets. All spectra were collected using a resolution of 2 cm1, and the mean of 50 scans was averaged in all the cases.

A suitable variable temperature cell Specac P/N 21525, with a pair of NaCl windows for transmission studies, was used to record the FT-infrared spectra at different temperatures. The cell consisted of a

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Fig. 1. Chemical structure of DHTSxT, DHSxT, DMSxT and DPTQtT. Atom numbering corresponds to those appearing in the paper. Bond numbering appears into circles and correspond to those ofFig. 7.

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surrounding vacuum jacket, with a combination of a refigerant dewar and a heatable block as the sample holder. It was fitted to a copper–constanton thermo-couple for temperature monitoring purposes, and any temperature ranging from 83 to 523 K could be achieved.

DFT calculations, for the neutral and doped mole-cules, were carried out with the Gaussian 98 program running on a SGI Origin 2000 supercomputer [31]. The standard 3-21G basis set was used in all the calculations, as a good compromise between accuracy and applicability to large systems. The 3-21Gbasis set, which includes a set ofd-symmetry polarization functions for the second-row elements[32], was used in conjunction with the B3LYP functional. In several studies, it has been shown that the B3LYP functional yields similar geometries for medium-sized molecules as MP2 calculations do with the same basis set[33]. DFT quadratic molecular force fields calculated with the B3LYP functional yield infrared absorption spec-tra in very good agreement with experiments[34–36]. Previous geometry optimizations were performed on isolated entities. Because of the long computing time of the force field calculations, only the all-anti copla-nar conformations were evaluated analytically within the same theoretical scheme used for the geometry optimizations. No scaling factors of the force con-stants were used and the theoretical frequencies were directly compared with the experiments.

3. General considerations

No experimental X-ray or electron diffraction data are available for DHSxT and DHTSxT. Supposedly, as shown by the X-ray structures of some related com-pounds (such as the unsubstituteda-linked oligothio-henes[37],a,a0-dimethyl end-capped quaterthiophene [38] and a,a0-dihexyl end-capped quaterthiophene [39]), it can be assumed that (a) the thienyl sulfur atoms are located in an all-anti configuration with respect to the long molecular axis and; (b) the whole molecule retains a nearly coplanar conformation of the aromatic units. With such a molecular structure the two hexamers belong to the C2h symmetry point

group. Nonetheless, the outermost rings of the oli-gothiophene chain possibly display a slight bent rela-tive to the inner rings least-square plane, and the strict

C2h symmetry could be partially broken. In what

follows, however, it will be assumed that both mole-cules present internal symmetry in solid state. As for DHTSxT, there exist 246 normal vibrational modes, 123 of them are IR-active and the remainder 123 Raman-active, as derived from the optical selection rules for the C2hsymmetry point group.

Although the optical selection rules predict a very large population of bands, both in the infrared and Raman spectra, the actual spectral patterns are fairly simple. This seeming discrepancy between theoretical predictions and experimental observations needs to be accounted for:

(i) since the side chains at the enda,a0-positions of the oligothiophene spine are far apart, no mech-anical coupling is expected to occur between their characteristic vibrations, and their in-phase and out-of-phase motions are expected to be fully degenerate, thus not showing any splitting in the spectra;

(ii) it is reasonable to believe that a sexithiophene chain is not large enough to observe progression of bands (i.e. set of close bands with frequency differences of about 4–5 cm1) associated to the same type of oscillators but with different phase angles;

(iii) usually, the infrared and Raman spectra of the

p-conjugated organic materials show for some few skeletaln(CC) stretching vibrations a selective intensity enhancement and sizeable frequency and intensity dispersions with variable number of units in the chain. This singular spectral feature has been explained by the existence of a very large electron–phonon coupling between the p-electrons system and some molecular vibrations with a pronounced collective char-acter[1,2].

4. Infrared spectra of the neutral molecules

The infrared spectra of DHSxT and DHTSxT in the high and medium low energy region are plotted in Figs. 2 and 3, respectively (the infrared spectrum of DMSxT has been also included)[30].Fig. 4compares the theoretical B3LYP/3-21G infrared spectrum of DPTQtT with the experimental one for DHTSxT in the

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1600–900 cm1 frequency range. Fig. 5 shows the eigenvectors associated to the stronger infrared absorptions of DPTQtT, while Table 1 summarizes a correlative analysis of frequencies measured in the infrared spectra of neutral DMSxT, DHSxT, and DHTSxT, as solids, together with their tentative assignment.

The infrared spectra of DHSxT and DHTSxT show characteristic absorptions around 3080–3060 cm1 assignable to aromatic n(C–H) stretching vibrations and four well resolved peaks below 3000 cm1, cor-responding to aliphaticn(C–H) stretchings. The broad features at 3061 cm1in DHSxT and at 3065 cm1in DHTSxT are due to stretchings of the C–H bonds attached at theb-positions of the inner rings[30,40]. On the other hand, the infrared band at 3078 cm1in DHSxT and at 3080 cm1in DHTSxT can be assigned

to stretching vibrations of the C–H bonds attached at the b-positions of the outermost thiophene rings [30,40]. Absorptions below 3000 cm1appear at the same frequencies in both compounds and have almost the same relative intensities. Band at 2955 cm1arises from antisymmetric stretching vibrations of the methylene groups of the hexyl side chains, na(CH2),

probably coupled to some extent with the antisym-metric stretching of the methyl end group,na(CH3). On

the other hand, bands at 2873 and 2854 cm1 are assignable to symmetric stretchings of the methylene groups,ns(CH2), also coupled with the corresponding

ns(CH3)[30,40].

The spectral region 1550–1350 cm1is overwhel-mingly dominated by the appearance of two or three bands. The band at 1492 cm1in DHTSxT could be correlated with the theoretical absorption of DPTQtT

Fig. 2. FT-IR spectra over probe energies of 3200–2800 cm1of neutral DHTSxT, DHSxT and DMSxT. Infrared spectrum of DMSxT has been taken from[30].

Fig. 3. FT-IR spectra over probe energies of 1600–400 cm1of neutral DHTSxT, DHSxT and DMSxT. Infrared spectrum of DMSxT has been taken from[30].

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at 1528 cm1(Fig. 4). Its associated eigenvector can be described as an antisymmetric stretching mode of the aromatic C=C bonds,na(C=C), spreading over the

whole oligothiophene chain (Fig. 5). The correspond-ing na(C=C) vibration in DHSxT is measured at

1503 cm1, on the basis of the B3LYP/3-21G eigen-vectors for DPQtT (being calculated at 1545 cm1). On the other hand, thisna(C=C) vibration is recorded

at the same frequency, 1503 cm1, both in DHSxT and DMSxT, thus confirming that the length of the alkyl side chain has a little influence on the vibrations of the

p-conjugated skeleton[30,40].

Let us pay some attention to the difference in frequency,Dn¼11 cm1, for a same type of skeletal vibration between DHTSxT and DHSxT.Fig. 6 com-pares the B3LYP/3-21GMu¨lliken atomic charges and bond lengths for the outermost rings of the oligothio-phene chain in DPQtT and DPTQtT (refer toFig. 1for atom numbering). The C11atomic charge varies from 0.21ein DPQtT to0.46ein DPTQtT, while those on the C10 and C9 atoms go from þ0.004e and

þ0.003e to 0.005e andþ0.020e, respectively. On the other hand, the C10–C11 and C9–C10 bonds

partially lose double and single bond character, respectively, upon attaching sulfur atoms at the end

a,a0_{-positions of the oligothiophene. These theoretical}

data could be explained by the balance between two resonant structures, mainly located over the outermost rings of the chain (Scheme 1)[41]. Under this hypoth-esis, the bond connecting atoms C10and C11 should

particularly weaken in going from DPQtT to DPTQtT, thus explaining the downshift by 11 cm1of the afore-mentioned na(C=C) vibration. The balance between

these two resonant structures should also induce certain degree of polarization of the conjugated bonds, parti-cularly of those of the outermost molecular domains.

Fig. 4. Comparison of: (a) infrared spectrum of neutral DHTSxT and (b) theoretical B3LYP/3-21G infrared spectrum of neutral DPTQtT.

Table 1

Frequency correlation of the main bands recorded in the infrared spectra of the neutral DMSxT, DHSxT and DHTSxT together with their assignment

DMSxT DHSxT DHTSxT Assignmenta

1503 1503 1492 na(C=C)

1466 1466 na(C=C)þda(CH2) 1457 1455 na(C=C)þda(CH2)

1443 1441 1444 ns(C=C)

1400 1429 ns(C=C)þds(CH2)

1370 1377 1383

1351

1313 n(C–C)intra-ring

1271 1274 1259 da(CH)

1245 1242 da(CH)

1222 1222

1203 1204 1207 n(C–C)inter-ring

1162 1166

da(CH)

1069 1069 1077 da(CH)

1044 1048 1069 da(CH)

989 n(C–S)alkyl 906

875 871 873 na(C–S)

837 840 838 ns(C–S)

795 791 795 g(CH)

683 725

668 668 dring

625

462 463 466 gring

a_n

, stretching; d, in-plane deformation; g, out-of-plane deformation.

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This fact could justify the appreciable infrared activity of the aliphaticn(C–S) stretchings at both chain ends, measured at 989 cm1and calculated at 1005 cm1, in spite of their low statistical weight as compared with the very many aromaticn(C=C),n(C–C) andn(C–H) modes of the inner thiophene units. The eigenvector

for the 1005 cm1B3LYP/3-21G infrared band of DPTQtT and the absence of the corresponding coun-terpart in the experimental spectrum of DHSxT sup-port the above assignment of the 989 cm1band.

Band measured at 1444 cm1in DHTSxT may be correlated with that calculated at 1453 cm1 for

Fig. 5. Schematic eigenvector for the more relevant infrared active vibrations of neutral DPTQtT calculated at the B3LYP/3-21Glevel (frequency values are given in cm1).

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DPTQtT, being described as a symmetric stretching vibration of the aromatic C=C bonds,ns(C=C), mostly

localized on the outer rings and where both end rings vibrate in full out-of-phase (seeFig. 5for the corre-sponding eigenvector).

The doublets at 1466 and 1457 cm1in DHSxT and at 1466 and 1455 cm1 in DHTSxT are new with respect to DMSxT [30]. These absorptions could be correlated with the theoretical band for DPTQtT at 1471 cm1, due to ans(C=C) mode mainly located on

the inner rings of the chain for which the motions of the symmetry-equivalent thiophene units also take place in full out-of-phase (Fig. 5).

The weak band at 1313 cm1in DHTSxT could be assigned to an antisymmetric stretching mode of ring C–C bonds,na(C–C). Weak bands at 1280–1230 cm1

are due to antisymmetric in-plane C–H deformations,

da(C–H), [30,40] whereas doublets at 1068 and

1048 cm1 in DHSxT and 1077 and 1068 cm1 in DHTSxT correspond to symmetric in-plane C–H deformations, ds(C–H), for which the motions of

the symmetry-equivalent oscillators occur in full out-of-phase. The doublets at 1222 and 1204 cm1 in DHSxT and the band at 1207 cm1in DHTSxT are mainly due to inter-ring CC stretching vibrations.

In the low energy region, bands at 871 and 840 cm1 in DHSxT, and 873 and 838 cm1 in DHTSxT are due to antisymmetric and symmetric aromatic n(C–S) stretchings, respectively, while the characteristic out-of-plane bending vibration of the 2,5-disubstituted thiophenes is easily identified with the band at 791 cm1 in DHSxT and 795 cm1 in DHTSxT[30,40].

Finally, the bands at 650–750 cm1in DHSxT and DHTSxT could be assigned to in-plane thiophene ring deformation vibrations, dring, whereas the bands at 463 cm1in DHSxT and 468 cm1in DHTSxT have been assigned to out-of-plane thiophene ring folding modes,gring[30,40].

5. Doped molecules

5.1. Molecular geometry optimizations and charges

Fig. 7shows the evolution of the calculated B3LYP/ 3-21G and UB3LYP/3-21G CC bond lengths on going from the neutral to the radical cationic forms of DPTQtT (detailed values are given in Table 2). Table 3 reports the Mu¨lliken atomic charges for

Fig. 6. Relevant Mulliken atomic charges (upper) and bond distances (below) calculated at the B3LYP/3-21Glevel for the outermost ring of neutral DPQtT and DPTQtT.

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Fig. 7. Optimized CC bond lengths of neutral DPTQtT (filled squares) and of DPTQtT as radical cation (open circles). The B3LYP and UB3LYP methods have been used for the close and open shell systems, respectively. SeeFig. 1for bond numbering.

Table 2

Bond lengths (in A˚ ) calculated at the B3LYP/3-21Gand UB3LYP/ 3-21G level for, respectively, the neutral and the radical cation systems of DPQtT and DPTQtT

DPQtT DPTQtT

Bond Neutral Radical cation

C1–C3 1.440 1.406 C1–C3 1.440 1.410

C3–C4 1.383 1.412 C3–C4 1.383 1.408

C4–C5 1.418 1.388 C4–C5 1.417 1.390

C5–C6 1.383 1.411 C5–C6 1.383 1.408

C6–C8 1.443 1.416 C6–C8 1.441 1.414

C8–C9 1.380 1.398 C8–C9 1.379 1.399

C9–C10 1.427 1.407 C9–C10 1.425 1.402

C10–C11 1.373 1.398 C10–C11 1.377 1.397

C11–S12 1.758 1.732 C11–C13 1.511 1.508 S12–C13 1.839 1.845 C13–C14 1.549 1.550 C13–C14 1.543 1.544 C14–C15 1.549 1.547 C14–C15 1.549 1.549

SeeFig. 1for bond numbering.

Table 3

Mulliken atomic charges calculated at the B3LYP/3-21G and UB3LYP/3-21Glevel for, respectively, the neutral and the radical cation systems of DPQtT and DPTQtT

DPQtT DPTQtT

Atom Neutral Radical cation

S2 0.46 0.54 S2 0.46 0.52

C3 0.25 0.25 C3 0.25 0.25

C4 0.01 0.07 C4 0.01 0.07

C5 0.01 0.07 C5 0.01 0.06

C6 0.25 0.24 C6 0.25 0.24

S7 0.43 0.51 S7 0.45 0.52

C8 0.25 0.26 C8 0.26 0.26

C9 0.003 0.006 C9 0.02 0.07

C10 0.004 0.004 C10 0.005 0.04

C11 0.21 0.19 C11 0.46 0.45

S12 0.31 0.40

C13 0.003 0.04 C13 0.09 0.08

C14 0.02 0.02 C14 0.01 0.03

C15 0.02 0.05 C15 0.04 0.07

SeeFig. 1for atom numbering.

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DPQtT and DPTQtT in their neutral forms (B3LYP/3-21G) and as radical cations (UB3LYP/3-21G). The main geometry changes upon ionization concern the distorsions of the p-conjugated C=C/C–C bonds, together with the C11–S12 bond in DPTQtT. The

optimized geometries for both radical cations indicate the generation of a positive polaron type defect over the quaterthienyl moiety in DPQtT, which further extends towards the C11–S12 bonds in DPTQtT. The

amplitude of the structural modifications progres-sively decrease away from the center of the molecules, however, in the case of the DPTQtT radical cation the C11–S12bond significantly shortens by 0.26 A˚ . This is

a large change as compared with those undergone by the inner CC bond lengths (center of the charged defect), whose greatest differences amount 0.30 A˚ . The analysis of the atomic charges also shows a large participation of the sulfur atoms in the stabilization of the positive polaron type defect. Thus, the atomic charges on the S2and S7atoms in DPQtT and DPTQtT

increase by 0.07e, while those on the S12a-linked

atom increase by0.09e.

5.2. Infrared spectra

Fig. 8 shows the experimental infrared spectra of the neutral and iodine-doped forms of DHSxT together with the UB3LYP/3-21Ginfrared spectrum of DPQtT as radical cation.Fig. 9 displays the same comparison as Fig. 8, but between the DHTSxT compound and its DPTQtT model system. Finally, Table 4summarizes the frequencies measured in the spectra of the two iodine-doped samples, and their tentative assignment.

In general terms, there exists a good agreement between experiments and calculations, what supports the reliability of the molecular parameters discussed along the preceding section. The infrared spectra of the doped molecules are characterized by the appear-ance of five intense infrared bands in the 1400– 1000 cm1 spectral region both in the experimental as in the theoretical spectra. The injection of a positive charge in the molecule give rise to strong charge fluxes along the p-conjugated backbone, generating strong infrared absorptions.

The infrared spectrum of iodine-doped DHSxT shows intense bands at 1401 and 1339 cm1, which are easily related with the theoretical features at 1437

and 1396 cm1. In the case of iodine-doped DHTSxT, experimental bands at 1399, 1365 and 1319 cm1are to be compared with the theoretical features at 1431, 1390 and 1264 cm1, respectively. Figs. 10 and 11 depict the eigenvectors associated to each of these theoretical infrared bands. All of these vibrations correspond ton(CC) stretching modes of the p -con-jugated backbone, mainly located in the transition region between the inner part of the chain (i.e. a molecular domain characterized by a quinonoid sin-gle–double bond alternation pattern) and the end thiophene rings (with a typical aromatic single–double bond alternation pattern)[6,7]. Please, note the sig-nificant contribution of thea-linked CS bonds to the molecular vibration associated to the band at 1264 cm1 in DPTQtT, what could justify for the different spectral patterns of the iodine-doped DHSxT and DHTSxT samples.

Fig. 8. Comparision of: (a) infrared spectrum of neutral DHSxT; (b) theoretical UB3LYP/3-21G infrared spectrum of DPQtT as radical cation; (c) infrared spectrum of iodine oxidized DHSxT.

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Fig. 9. Comparision of: (a) infrared spectrum of neutral DHTSxT; (b) theoretical UB3LYP/3-21Ginfrared spectrum of DPTQtT as radical cation; (c) infrared spectrum of iodine oxidized DHTSxT.

Table 4

Correlation between the vibrational frequencies measured in the infrared spectra of iodine-oxidized DHSxT and of iodine-oxidized DHTSxT

DHSxT-I2 DHTSxT-I2 Assignment

1453 1455 –

1401 1399 n(CC)

– 1365 n( C C ) þ

n(CS)0

1339 – n(CC)

– 1319 n( C C ) þ

n(CS)0

– 1249 –

1213 1226 –

1174 1169 –

1110 1106 –

1095 – –

– 1070 –

1041 – –

1014 – –

996 – –

– 955 n(CS)0

892 883 –

842 837 –

788 791 –

724 723 –

670 679 –

455 465 –

n(CS)0_{: alkyl side CS stretching vibration.}

Fig. 10. Schematic eigenvectors for the most intense infrared bands of the theoretical UB3LYP/3-21Gspectrum of DPQtT as radical cation (all values are in cm1).

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The spectrum of doped DHTSxT shows a strong band at 955 cm1, which is missing in the spectrum of doped DHSxT. This band must be correlated with the experimental feature at 989 cm1in neutral DHTSxT, being likely associated with an aliphaticn(CS) stretch-ing mode of the doped material. The reason for the strong infrared-activity of this vibration can be found in the polarization of the a-linked C–S bonds, as shown by the B3LYP/3-21GMu¨lliken atomic charges of the neutral and radical cationic forms of DPTQtT. Fig. 12shows the infrared spectra of iodine-doped DHTSxT at 1708C and at room temperature. The band at 1319 cm1 becomes stronger on lowering the temperature. Different authors have concluded that thep-conjugated oligothiophene backbone reaches

a more planar conformation of the thiophene rings at low temperatures[43]. In this regard, we believe that the increasing conformational order of the thioalkyl side chains, at low temperatures, should also lead to a more favorable overlapping between the d-type orbi-tals of the a-linked S atoms and the p-conjugated backbone, thus increasing the participation of the end thioalkyl groups in the stabilization of the radical cation (which in its turn is reflected in a stronger polarization of the C–S bond, and the subsequent intensification of the infrared band at 1319 cm1).

The infrared absorptions of doped DHTSxT are somewhat downshifted with respect to those of doped DHSxT. These observations can be rationalized within the framework of the effective conjugation coordinate

Fig. 11. Schematic eigenvectors for the most intense infrared bands of the theoretical UB3LYP/3-21Gspectrum of DPTQtT as radical cation (all values are in cm1).

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theory (ECC) developed by Zerbi and coworkers to explain the simple appearance of the Raman spectral patterns of undoped p-conjugated materials and the upsurge of strong and broad absorptions in the infrared spectrum upon chemical doping or photoexcitation [44]. These authors correlate the doping-induced infrared bands with initially silent totally symmetric normal modes with a large contribution of a particular vibrational coordinate, usually termed as ECC coor-dinate, which become activated in the infrared due to the breakdown of the optical selection rules in the molecular domain perturbed by the doping process. The ECC coordinate describes a collective vibration of thep-conjugated path along which all the C=C bonds lengthen phase while all the C–C bonds shrink in-phase. Thus, the ECC skeletal vibration points in the direction from a benzenoid structure (usually that of ground state) to a quinonoid structure (usually that

of the first electronically excited state or of the charged defect).

During the oxidation of a p-conjugated material, ring C=C bonds are weakened, while inter- and intra-ring C–C bonds are strengthened. Therefore, with respect to the neutral form, normal modes of a doped or photoexcited species with large contents ofn(C=C) stretchings shift downward due to the softening of the double C=C bonds (i.e. specially in the case of mole-cular vibrations with a large contribution of the col-lective ECC vibrational coordinate). In the Raman spectrum of neutral DHTSxT the strongest line, appearing at 1458 cm1, arises from a totally sym-metric normal mode whose associated eigenvector greatly remembers the ECC coordinate [28]. On the other hand, the strongest infrared absorption of iodine-doped DHTSxT assignable to a n(C=C) stretching vibration is that measured at 1319 cm1. Thus size-able downshifts (by even more than 100 cm1) are observed to take place upon the partial quinoidization of the sexithiophene spine, in full agreement with the statements of the ECC theory.

Neutral polythiophene exhibits, in the 800– 1600 cm1spectral range, four Raman-activeAg

nor-mal vibrations, which give rise upon chemical doping or photoexcitation to an infrared absorption pattern with three main components at 1319, 1195 and 1090– 1060 cm1 [42,45,46]. These spectroscopic data are quite similar to those reported in this paper for iodine-doped DHTSxT. ECC theory states that the strong doping-induced infrared bands should downshift as the chain length (or conjugation length) of the oligo-mers grows up. As aforementioned, the doping-induced infrared bands of DHTSxT appear at lower frequencies than in DHSxT, but the oligothiophene backbone has the same chain length in both com-pounds. The most feasible explanation is the strong participation of thea-linked sulfur atoms in the sta-bilization of the doped species, being the positive charge delocalization in iodine-doped DHTSxT simi-lar to that of a photoexcited or doped polythiophene sample.

6. Conclusions

A comprehensive study of the infrared vibrational properties of two sexithiophenes, with their end

Fig. 12. Infrared spectrum of iodine-oxidized DHTSxT recorded at: (a) room temperature; (b)1708C.

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a,a0-positions capped byn-hexyl or -thiohexyl groups, in neutral state has been reported. A tentative assign-ment of the main infrared spectral features of the corresponding iodine-doped species is also proposed. The different spectral behavior of the two hexamers has been interpreted, at the light of the statements of the ECC theory of Zerbi’s group, in terms of the role played by thea-linked sulfur atoms in the overall

p-conjugation of the undoped molecule and the stabilization of the oxidized forms.

The present study shows that upon oxidation of DHSxT and DHTSxT with iodine vapors, a radical cation species is generated. The vibrational infrared features of these radical cation species can be used to identify prototypes of polaron-like charged defects in other classes of p-conjugated thiophene-based mole-cular materials. The comparison of the doping-induced infrared absorptions of iodine-doped DHTSxT and DHSxT with the infrared spectra of doped or photo-excited polythiophene has led to the conclusion that the electron-donating effect of the enda-thiohexyl groups improve the delocalization alongthechainof the positive charge injected in the doping process. The analysis of the infrared data is consistent with the Raman data previously reported on neutral and electrochemically doped DHSxT and DHTSxT.

In terms of molecular electronics, a-linked sulfur atoms seem to be good candidates to act as alligator clips, thus preserving the oligothiophene backbone from electronic interactions with a macroscopic sur-face. In addition, sulfur atoms facilitate the connec-tions of this type ofp-conjugated molecular material with a metallic or cluster surface, strongly stabilizing the radical cations which are likely present in the charge-separated state of the operating forms of the molecular electronic devices.

Acknowledgements

The present work was supported in part by the Direccioń General de Ensenãnza Superior (DGES, MEC, Spain) through the research projects BQU2000–1156 and FD97–1765–C03. We are also indebted to Junta de Andalucıá (Spain), funding for our research group (FQM–0159). J.C. is grateful to the Ministerio de Educacioń y Cultura of Spain for a PostDoctoral fellowship at the Department of Chemistry

of the University of Minnesota (Formacio´n y Perfec-cionamiento de Doctores y Tecno´logos en el Extranjero, referencia PF00 25327895).

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